Longitudinal waves are waves in which the displacement of the medium is in the same (or opposite) direction of the wave propagation. Mechanical longitudinal waves are also called ''compressional'' or compression waves, because they produce
compression and
rarefaction when traveling through a medium, and pressure waves, because they produce increases and decreases in pressure. Sound travels through longitudinal waves. Sound travels through transversal waves in 90 degrees.
The other main type of wave is the
transverse wave, in which the displacements of the medium are at right angles to the direction of propagation. Transverse waves, for instance, describe ''some'' bulk sound waves in solid materials (but not in fluids); these are also called "shear waves" to differentiate them from the (longitudinal) pressure waves that these materials also support.
Longitudinal waves include sound waves (
vibrations in pressure, a particle of displacement, and particle velocity propagated in an
elastic medium) and seismic
P-waves (created by earthquakes and explosions).
In longitudinal waves, the displacement of the medium is parallel to the propagation of the wave. A wave along the length of a stretched
Slinky toy, where the distance between coils increases and decreases, is a good visualization and contrasts with the
standing wave along an oscillating guitar string which is transverse.
Nomenclature
"Longitudinal waves" and "transverse waves" have been abbreviated by some authors as "L-waves" and "T-waves", respectively, for their own convenience. While these two abbreviations have specific meanings in
seismology (L-wave for
Love wave or long wave) and
electrocardiography (see
T wave), some authors chose to use "l-waves" (lowercase 'L') and "t-waves" instead, although they are not commonly found in physics writings except for some popular science books.
Sound waves
In the case of longitudinal harmonic sound waves, the
frequency and
wavelength can be described by the formula
:
where:
* ''y'' is the displacement of the point on the traveling sound wave;
305px|Representation of the propagation of an omnidirectional pulse wave on a 2d grid (empirical shape)
* ''x'' is the distance the point has traveled from the wave's source;
* ''t'' is the time elapsed;
* ''y''
0 is the
amplitude of the oscillations,
* ''c'' is the speed of the wave; and
* ''ω'' is the
angular frequency of the wave.
The quantity ''x''/''c'' is the time that the wave takes to travel the distance ''x''.
The ordinary frequency (''f'') of the wave is given by
:
The wavelength can be calculated as the relation between a wave's speed and ordinary frequency.
:
For sound waves, the amplitude of the wave is the difference between the pressure of the undisturbed air and the maximum pressure caused by the wave.
Sound's
propagation speed depends on the type, temperature, and composition of the medium through which it propagates.
Pressure waves
The equations for sound in a fluid given above also apply to acoustic waves in an elastic solid. Although solids also support transverse waves (known as
S-waves in
seismology), longitudinal sound waves in the solid exist with a
velocity and
wave impedance dependent on the material's
density and its
rigidity, the latter of which is described (as with sound in a gas) by the material's
bulk modulus.
Electromagnetics
Maxwell's equations lead to the prediction of
electromagnetic waves in a vacuum, which is strictly
transverse waves, that is, the electric and magnetic fields of which the wave consists are perpendicular to the direction of the wave's propagation.
[David J. Griffiths, Introduction to Electrodynamics, ] However
plasma waves are longitudinal since these are not electromagnetic waves but density waves of charged particles, but which can couple to the electromagnetic field.
After
Heaviside's attempts to generalize
Maxwell's equations, Heaviside concluded that electromagnetic waves were not to be found as longitudinal waves in "''
free space''" or homogeneous media. Maxwell's equations, as we now understand them, retain that conclusion: in free-space or other uniform isotropic dielectrics, electro-magnetic waves are strictly transverse. However electromagnetic waves can display a longitudinal component in the electric and/or magnetic fields when traversing
birefringent materials, or inhomogeneous materials especially at interfaces (surface waves for instance) such as
Zenneck waves.
In the development of modern physics,
Alexandru Proca (1897-1955) was known for developing relativistic quantum field equations bearing his name (Proca's equations) which apply to the massive vector spin-1 mesons. In recent decades some other theorists, such as
Jean-Pierre Vigier and Bo Lehnert of the Swedish Royal Society, have used the Proca equation in an attempt to demonstrate photon mass
as a longitudinal electromagnetic component of Maxwell's equations, suggesting that longitudinal electromagnetic waves could exist in a Dirac polarized vacuum. However
photon rest mass is strongly doubted by almost all physicists and is incompatible with the
Standard Model of physics.
See also
*
Transverse wave
*
Sound
*
Acoustic wave
*
P-wave
*
Plasma waves
References
Further reading
* Varadan, V. K., and Vasundara V. Varadan, "''Elastic wave scattering and propagation''". ''Attenuation due to scattering of ultrasonic compressional waves in granular media'' - A.J. Devaney, H. Levine, and T. Plona. Ann Arbor, Mich., Ann Arbor Science, 1982.
* Schaaf, John van der, Jaap C. Schouten, and Cor M. van den Bleek, "''Experimental Observation of Pressure Waves in Gas-Solids Fluidized Beds''". American Institute of Chemical Engineers. New York, N.Y., 1997.
*
*
* Russell, Dan, "
Longitudinal and Transverse Wave Motion'". Acoustics Animations, Pennsylvania State University, Graduate Program in Acoustics.
* Longitudinal Waves, with animations "
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